1. Field of the Invention
The present invention relates to a process for producing an aqueous solution of a mixture of a straight-chain alkyl tertiary amine oxide and a branched-chain alkyl tertiary amine oxide by reacting aliphatic tertiary amines with an aqueous solution of hydrogen peroxide.
2. Prior Art
Aliphatic tertiary amine oxides are effective surface active agents and are extensively incorporated in shampoos and dish detergents so as to formulate liquid detergents that have mild effects on the hand. However, amine oxides suitable for use in this application must have a minimum content of unreacted amines. The presence of large quantities of unreacted amines not only reduces the cleaning effect of the amine oxides but also causes undesired effect on the color, smell and skin irritating property of the final product. Therefore, at least 99 mol % conversion is necessary for producing commercially acceptable amine oxides by reacting tertiary amines with hydrogen peroxide.
A common practice for reducing the residual amount of unreacted amines is by reacting amines with excess hydrogen peroxide. However, the amine oxide that can be incorporated in liquid detergents for home use should preferably be free of residual hydrogen peroxide, and after completion of the reaction, the residual hydrogen peroxide must be decomposed to a level of 0.1 wt % or less. However, oxygen gas evolving during the decomposition of hydrogen peroxide causes extensive bubbling of the aqueous amine oxide solution, and this prevents the smooth progress of the decomposition reaction.
Several methods have been proposed for producing amine oxides by using a small excess of hydrogen peroxide so as to reduce the residual hydrogen peroxide level to 0.1 wt % or below and to achieve at least 99 mol % conversion of the amine. According to Japanese Patent Publication No. 14089/1966, the amine is reacted with hydrogen peroxide in the presence of a chelating agent that is selected from among diethylenetriamine pentaacetic acid, ammonium salts and alkali metal salts thereof. Japanese Patent Publication No. 11042/1967 shows a method wherein sodium pyrophosphate and sodium bicarbonate are simultaneously added to the reaction system. Japanese Patent Application (OPI) No. 54160/1982 (the symbol OPI as used herein means an unexamined published Japanese patent application) proposes performing the reaction in the presence of a polybasic acid having one or more hydroxyl groups, or a salt of such polybasic acid.
Detergents comprising a mixture of a tertiary amine oxide having a long, straight-chain alkyl group and tertiary amine oxide having a long, branched-chain alkyl group are known to be superior to the detergents containing above described aliphatic tertiary amine oxide with respect to the non-skin irritating effect, stability at low temperature, solubilizability and detergency (see Japanese Patent Publication No. 19526/1966, Japanese Patent Application (OPI) Nos. 141400/1981 and 139200/1982)).
The mixed tertiary amine which provides a mixture of branched-chain alkyl tertiary amine oxide and straight-chain alkyl tertiary amine oxide may be prepared by the following process: first, a higher alcohol containing a mixture of branched-chain and straight-chain alkyls that is synthesized by the oxo process is converted to a mixed alkyl chloride, which is then aminated with a lower dialkylamine. Alternatively, as shown in Japanese Patent Application (OPI) No. 105945/1983, a long-chain olefin may be reacted with a lower dialkylamine in the presence of a gaseous mixture of hydrogen and carbon monoxide. The mixed tertiary amines produced by these methods have a straight chain content in the range of 40 to 80 wt %.
One may assume that in producing an amine oxide by oxidizing the mixture of branched-chain and straight-chain alkyl tertiary amines with hydrogen peroxide, the level of residual hydrogen peroxide could be reduced to 0.1 wt % or less whereas the conversion of the mixed amine be increased to 99 mol % or higher by direct application of one of the methods that are described above in connection with the production of the aliphatic tertiary amine oxide. As it turned out, however, these objects were very difficult to attain. The rate of reaction of the branched-chain alkyl tertiary amine is much slower than that of the straight-chain alkyl tertiary amine, and even if the conversion of the straight-chain alkyl tertiary amine has reached 100%, much of the branched-chain alkyl tertiary amine remains unreacted. In order to enhance the conversion of the branched-chain alkyl tertiary amine, an excess amount of hydrogen peroxide must be used, but then, the other object, i.e., reduced level of residual hydrogen peroxide, cannot be obtained.